UV photoactivatable curable paint formulations for providing cured paint coatings on a substrate by UV irradiation are known. These prior art formulations comprise UV curable film-forming compounds and a UV photoinitiator to initiate polymeric reaction of the film-forming compounds under the action of the UV radiation.
Such formulations may be pigment-free if a clear coat is desired or contain pigments, dyes and the like if a white, coloured, metallic or other effect appearance is desired. Metallic paints comprising particulate metals are, thus, known wherein the particulate metal is present to provide a desired resultant appearance.
It will be readily understood that it is generally desirable that suitable curing of a curable paint formulation be attained in a reasonable period of time. This is extremely desirable in a commercial environment, such as the surface coating of vehicle bodies and parts thereof on a manufacturing assembly line where the rate of vehicle throughput in a paint shop can have a significant operational cost to the overall manufacturing process.
PCT/EPO5/005517, published Nov. 24, 2006, corresponding to US 2008/0032037A1, published Feb. 7, 2008—Frey, Thomas et al acknowledges that unlike their transparent counterparts, pigmented coating materials per se are difficult to cure by radiation, since the pigments they comprise absorb and reflect the radiation and hence only a small part of the irradiated energy does is actually able effectively to bring about curing. The use of radiation curing for colored and opaque coatings is therefore hindered by the interaction of the pigments used with the radiation, whose intensity is attenuated. Volume curing of the coating particularly at its underside, i.e., down to the substrate, can be reduced as a result of the pigmentation to the point where the coating becomes unusable. Further, aforesaid US 2008/0032037 A1 notes that there has been no lack of attempts to extend radiation curing to pigmented coating materials. Such attempts have involved exposing the coating materials to radiation for a duration empirical data suggested would lead to volume curing.
To address the aforesaid disadvantages, US 2008/0032037 A1 describes a method of allowing, on the one hand, the suitability or non-suitability of radiation curing to be predicted for a specified pigmentation of a coating and, on the other hand, allowing the variables for radiation curing to be determined in such a way that sufficient volume curing can be expected. That object is achieved by a method of determining the conditions for radiation curing radiation-curable pigmented coating materials comprising at least one pigment P, at least one binder B and at least one photoinitiator I on a substrate, comprising the steps of determining, in effect, a plurality of defined parameters.
A promised advantage of the aforesaid method is that the scope of experimental test series can be substantially reduced, the utilization of the exposure units can be optimized, and off-specification batches due to inadequate radiation can be avoided.
There is, however, always the need to enhance the rate of curing of a curable paint formulation irrespective of the nature of the curing process whether it be, for example, a chemical, thermal (baking), or radiation by UV, IR and/or microwave process.